New Metallic glass Steel composite three times more resistant to impacts than Tungsten Carbide

University of California San Diego researchers have a made new steel that has nearly three times the elastic limit of tungsten carbide. Tungsten carbide (a high-strength ceramic used in military armor) has an elastic limit of 4.5 giga-Pascals while SAM2X5-630 has a 12.5 gigapascal limit.

The new steel alloy could be used in a wide range of applications, from drill bits, to body armor for soldiers, to meteor-resistant casings for satellites.

Metallic glass steel properties can be customized for high strength applications.

SAM2X5-630 has the highest recorded elastic limit for any steel alloy, according to the researchers—essentially the highest threshold at which the material can withstand an impact without deforming permanently. The alloy can withstand pressure and stress of up to 12.5 giga-Pascals or about 125,000 atmospheres without undergoing permanent deformations.

Researchers at USC tested how the alloy responds to shock without undergoing permanent deformations by hitting samples of the material with copper plates fired from a gas gun at 500 to 1300 meters per second. The material did deform on impact, but not permanently.

Transmission electron microscopy image showing different levels of crystallinity embedded in the amorphous matrix of the alloy.

The Hugoniot Elastic Limit (the maximum shock a material can take without irreversibly deforming) of a 1.5-1.8 mm-thick piece of SAM2X5-630 was measured at 11.76 ± 1.26 giga-Pascals.

Stainless steel has an elastic limit of 0.2 giga-Pascals. Diamonds top out at a whopping 60 giga-Pascals— they’re just not practical for many real-world applications. “The fact that the new materials performed so well under shock loading was very encouraging and should lead to plenty of future research opportunities,” said Eliasson.
The primary focus of future research efforts on these alloys is increasing the weight of the materials to make them more resistant to impacts.

The primary focus of future research efforts on these alloys is increasing the weight of the materials to make them more resistant to impacts.

To make the solid materials that comprise the alloy, Graeve and her team mixed metal powders in a graphite mold. The powders were then pressurized at 100 mega-Pascals, or 1000 atmospheres, and exposed to a powerful current of 10,000 Ampers at 1165°F (630°C) during a process called spark plasma sintering.

The spark plasma sintering technique allows for enormous time and energy savings, Graeve said. “You can produce materials that normally take hours in an industrial setting in just a few minutes,” she said.

The process created small crystalline regions that are only a few nanometers in size, with hints of structure, which researchers believe are key to the material’s ability to withstand stress. This finding is promising because it shows that the properties of these types of metallic glasses can be fine-tuned to overcome shortcomings such as brittleness, which have prevented them from becoming commercially applicable on a large scale, Eliasson said.

In conclusion, amorphous steels, the high-strain rate mechanical response of which was hitherto unexplored, demonstrate high strength under shock wave compression, the magnitude of the elastic limit of one amorphous steel composite being 1.5 times those reported previously for amorphous metals. Further, a minor addition of nanocrystallinity to the amorphous matrix of the iron-based BMG studied in this work results in a significant improvement in yield strength and post-yield shear strength retention, as seen in the response of the partially crystalline SAM2X5-630 when compared with that of the X-ray amorphous SAM2X5-600.

This result is in contrast with a previous work on Zr-BMG Vitreloy 1 and its in situ dendritic phase composite, where no difference was seen in the shock response of the monolithic BMG and its composite26. While the shock response of the amorphous steels of this work is qualitatively similar to that of previously studied Zr-based compositions, namely large amplitude elastic waves and loss of post-yield strength, a significant enhancement in response based on the extent of devitrification is observed in both of these attributes. This work therefore reveals in situ reinforced metallic glass composites as promising candidates for use in high-strength applications. Furthermore, they demonstrate that controlled devitrification is a potentially viable adjustable parameter for synthesis of amorphous metallic materials with properties that can be tailored as desired.

Abstract

The response of amorphous steels to shock wave compression has been explored for the first time. Further, the effect of partial devitrification on the shock response of bulk metallic glasses is examined by conducting experiments on two iron-based in situ metallic glass matrix composites, containing varying amounts of crystalline precipitates, both with initial composition Fe49.7Cr17.7Mn1.9Mo7.4W1.6B15.2C3.8Si2.4. The samples, designated SAM2X5-600 and SAM2X5-630, are X-ray amorphous and partially crystalline, respectively, due to differences in sintering parameters during sample preparation. Shock response is determined by making velocity measurements using interferometry techniques at the rear free surface of the samples, which have been subjected to impact from a high-velocity projectile launched from a powder gun. Experiments have yielded results indicating a Hugoniot Elastic Limit (HEL) to be 8.58 ± 0.53 GPa for SAM2X5-600 and 11.76 ± 1.26 GPa for SAM2X5-630. The latter HEL result is higher than elastic limits for any BMG reported in the literature thus far. SAM2X5-600 catastrophically loses post-yield strength whereas SAM2X5-630, while showing some strain-softening, retains strength beyond the HEL. The presence of crystallinity within the amorphous matrix is thus seen to significantly aid in strengthening the material as well as preserving material strength beyond yielding.